1. Technical Field
The present invention relates to an imaging system, a writing head and an image forming apparatus.
2. Background Art
Exposing devices used for image forming apparatuses such as laser printers and copiers include a writing head configured with a light source such as light emitting diode (LED) array and an organic electroluminescence (OEL) array, and a lens array. The lens array of the writing head may employ a gradient index lens array, but light use efficiency of the gradient index lens array may not be sufficient. Therefore, the gradient index lens array may not be employed for high speed apparatuses. Especially, if the OEL is used as the light source, because light quantity of the OEL is smaller than light quantity of the LED, an optical system to enhance light use efficiency is required.
To enhance the light use efficiency of the writing head, an imaging system configured with a lens and a roof prism or with a lens and a roof mirror can be used. In such imaging system, a plurality of optical systems are arranged in a main scanning direction, and the lens array pitch is same as the roof prism array pitch or the roof mirror array pitch. Therefore, this imaging system has a retroreflective optical system that reflects an image for two times in a main scanning direction using the roof prism and the roof mirror, and an “upright image” can be generated in the main scanning direction, and an “inverted image” can be generated in a sub-scanning direction.
In the above described imaging system, if some of the light is focused at a position on an imaging face that should not be focused, ghost light may occur. The ghost light can be suppressed by disposing a slit at the lens array side.
The ghost light can be reduced using the slit having an light absorbing effect, but the light use efficiency becomes lower due to the light absorbing by the slit.
Further, an aperture can be disposed at the incidence face side of the imaging system to suppress light propagation to undesired positions. However, it is very difficult to manufacture an aperture array matched to each incidence face of the imaging system with high precision. Therefore, the aperture may not be practical for preventing occurrence of ghost light.
Further, an aperture can be disposed at the exit face side of the imaging system to suppress occurrence of ghost light. The aperture disposed at the exit face side of the imaging system can be a single aperture, but the single aperture becomes a long aperture in a long side direction of the imaging system. Such long aperture is difficult to manufacture, and strength of the aperture becomes weak, and resultantly the aperture becomes weak to vibration. Therefore, mechanical strength of the imaging system becomes lower.
Further, the number of parts increases when the aperture is disposed as above described. Further, a correct positioning between the aperture and the optical face of the imaging system is required, which increases cost of the imaging system.